In recent years, light sources such as light emitting diodes (LEDs) have been improved through technological advances in material and manufacturing processes. The LEDs possess characteristics such as a relatively high efficiency, a relatively long life, and vivid colors, and can be used in a variety of industries. One example is to use the LEDs to replace traditional incandescent bulbs in a vehicle lamp. LEDs present advantages over incandescent light bulbs including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliance.
FIG. 1 shows a conventional LED driving circuit 100 for powering multiple LED strings in a vehicle tail light. LED strings 108_1, 108_2, . . . , 108_N are powered by a DC power source 102, e.g., a battery. Each LED string can be coupled to ground through two resistors connected in parallel. For example. LED string 108_1 is coupled to ground through a resistor 112_1 and a resistor 114_1. The resistor 112_1 and the resistor 114_1 are connected in parallel. The resistor 114_1 is coupled to a switch 110_1 in series. In operation, if the switch 110_1 is turned off, a current flows through the LED string 108_1 and the resistor 112_1 to ground. When a driver applies the brake of the vehicle, a dimming signal ADJ turns on the switch 110_1 such that the total resistance of the parallel-connected resistor 112_1 and resistor 114_1 is less than the resistance of the resistor 112_1. Therefore, the current flowing through the LED string 108_1 increases and the brightness of the LED string 108_1 increases. Similarly, when the driver applies the brake of the vehicle, the dimming signal ADJ turns on the switch 110_2, . . . 110_N such that the brightness of the LED strings 108_2, . . . 108_N increases. One of the drawbacks of the conventional LED driving circuit 100 is that due to different forward voltages of the LED strings, different LED strings may have different currents, so that the brightness of each LED string may be different.